Diaphragm type magnetic speakers have been known for several years, and usually incorporate a diaphragm or membrane having a vibratable area with a multiplicity of runs of signal carrying conductors thereon. The diaphragm or membrane is spaced from and confronts a generally rigid magnetic backing, usually comprising a multiplicity of permanent magnetized strips lying against an acoustically transparent soft iron plate or armature. The magnetized strips are magnetized so that the front face of each magnetized strip which faces the diaphragm has one polarity and the opposite face of the strip which faces the magnetic plate is the opposite polarity. The magnetized strips are spaced from each other and are magnetized so that all magnetized strips have polarity arrangements opposite to the polarity arrangement of the next adjacent strips. That is to say, the accumulation of spaced magnet strips are polarized so that the adjacent faces have the polarity arranged, north-south-north-south-etc.
Prior transducers have had various physical constructions in the magnetic backing. In U.S. Pat. No. 3,674,946, the magnetized strips are incorporated into a single sheet or slab of magnetic material which is variously magnetized in parallel zones or strips which are spaced from each other. In U.S. Pat. No. 3,919,499, the magnetic backing utilized narrow strips of magnet material, each strip spaced from adjacent strips and suitably magnetized.
In these prior speakers or transducers, conductors on the diaphragm extend parallel to the magnetized strips and are located opposite all the spaces between the several magnets in the magnetic backing.
The arrangement of the magnetized strips has been such that the magnetic fields are formed between the adjacent magnetic poles at the front faces of adjacent magnetized strips; and approximately half of the magnetic field of each magnetized strip is associated with the pole face of the next adjacent strip and the magnetic field related thereto.
It has been recognized in the past that one of the principle problems encountered in the use of the diaphragm type magnetic transducers or speakers has been one of efficiency. In order that the magnetic fields in the vicinity of the conductors on the diaphragm have sufficient strength as to produce significant vibration of the diaphragm in response to application of a signal current through the conductors, it has been necessary to locate the diaphragm quite close to the faces of the magnet strips. It has been typical practice to space the diaphragm approximately sixty thousandths (0.060 inches) from the faces of the magnetized strips in commercial diaphragm type magnetic speakers.
It has been experienced that when a signal current of a substantial magnitude is applied to the conductors on the diaphragm, the diaphragm may have a sufficient excursion from its normal position as to "bottom" or slap against the faces of the magnetized strips. Of course, this bottoming of the diaphragm against the magnetized strips causes a sound which is quite unpleasant and which does not conform at all to the sounds intended to be produced by the signal current being applied.
If the diaphragm is to be prevented from bottoming against the faces of the magnetized strips, the magnitude of the signal current must be reduced, in which case the volume of the sound produced may not be as large as may be desired; or on the other hand, the spacing between the diaphragm and the faces of the magnetized strips must be increased to the point wherein only a minimum of magnetic field surrounds the conductors so that the signal current in the conductors has a significantly lesser effect for the purpose of producing vibration of the diaphragm.